The news is based on a study that found tooth loss was associated with an increased risk of dementia.

The study involved more than 1,500 elderly people in Japan who had their health monitored between 2007 and 2012.

The study found participants with fewer teeth had a greater chance of developing dementia within the five years of the study.

For example, people with 1-9 teeth had an 81% higher risk of dementia than those with 20 teeth or more.

There are 850,000 people with dementia in the UK, with numbers set to rise to more than a million by 2025.

Although this isn't the first study to link oral hygiene with dementia, we don't know whether tooth loss is a cause of dementia or whether it could be a sign of something else.

Poor oral hygiene might be a sign of poor overall health or unhealthy behaviour, or it might be linked to having a poor diet – it's harder to eat a full, balanced diet if you don't have many teeth.

Although the study doesn't show that tooth brushing can "ward off" dementia, there are plenty of good reasons to keep teeth healthy.

Tooth decay not only causes pain, but chronic inflammation that's been linked to risk of heart disease.

Good oral hygiene includes brushing your teeth twice a day, regular visits to the dentist, and avoiding sugary food and drink.

Where did the story come from?

The study was carried out by researchers from Kyushu University in Japan, and was funded by the Ministry for Education, Culture, Sports, Science and Technology of Japan, the Ministry of Health, Labour and Welfare of Japan, and the Japan Agency for Medical Research and Development.

Despite the misleading headline, the Daily Express reported on the study reasonably accurately. The i newspaper and the Daily Mirror also did a reasonable job.

But the stories also said a study we reported on last year showed tooth brushing reduced dementia risk – when in fact all the people in last year's study already had dementia, and it looked at gum disease, not whether people brushed their teeth.

What kind of research was this?

This was a prospectivecohort study. Researchers wanted to compare what happened to people with different levels of tooth loss to see who was most likely to develop dementia.

These types of studies are useful for identifying links between factors, but can't tell us whether one factor (such as tooth loss) causes another (dementia).

What did the research involve?

Researchers followed 1,566 adults aged 60 or above in one region of Japan. They had their teeth checked by a dentist and were asked about many aspects of their lives.

They were followed up closely for five years (2007-12) to check for signs of dementia.

After adjusting for confounding factors, the researchers looked at whether people with less than 20 remaining teeth were more likely to have developed dementia of any type, compared with those with at least 20 teeth.

Diagnoses of dementia were made by specialist stroke physicians and psychiatrists. They aimed to differentiate between Alzheimer's disease and vascular dementia, which is caused by multiple small strokes that damage the brain.

Researchers adjusted figures to take account of a wide range of potential confounding factors, including people's age, sex, job, history of high blood pressure, stroke or diabetes, education levels, smoking and alcohol intake, tooth brushing frequency, use of dentures, and regular visits to the dentist.

They looked at the risk of getting any type of dementia, then at the risks of Alzheimer's disease and vascular dementia separately.

What were the basic results?

During the study, 180 people (11.5%) developed some type of dementia. Compared with people who had 20 teeth or more:

those with 1-9 teeth had an 81% higher risk of dementia (HR 1.81, 95% CI 1.11 to 2.94)

those with no teeth had a 63% higher risk of dementia, although this figure could have been down to chance, probably because of the small number of people in the study with no teeth (HR 1.63, 95% CI 0.95 to 2.80) – these results could also be affected by people with no teeth wearing a full set of dentures

The researchers found no link between number of teeth and vascular dementia. Although they found a link between numbers of teeth and Alzheimer's disease, this number didn't stand up after adjusting for confounding factors.

How did the researchers interpret the results?

The researchers said: "These findings highlighted the clinical value of maintaining healthy dentition [teeth] throughout life to reduce the risk of dementia in the general population."

They suggest several ways in which tooth loss may be linked to dementia risk.

They say the act of chewing might stimulate blood flow to the brain, or people with a full set of teeth may have a healthier diet, and inflammation from long-term tooth decay or gum disease might increase the likelihood of Alzheimer's disease.

They also admit that poor oral health could be a general sign of overall poor health.

Conclusion

This study adds to the evidence that good oral health is linked to good overall health, including a reduction in the chances of developing dementia in later life.

But the research doesn't prove that regular tooth brushing will prevent dementia.

We don't know what causes dementia. From research so far, it looks as if there are a number of interlinked causes.

Brain health and ageing are likely to be affected by factors including diet, exercise, smoking, alcohol use, blood pressure and genetics.

While living a healthy lifestyle may certainly reduce the chances of dementia, there are no guarantees.

This study has a few limitations. The number of people in the study, and the number who got dementia, was relatively small.

That means we should be cautious about the results, especially when looking at Alzheimer's disease and vascular dementia separately.

Only 42 people of 1,566 people had vascular dementia, so it's hard to draw conclusions based on such small numbers. That's why we say some of the results could be down to chance.

This type of study does not allow us to say whether the factors studied (tooth loss) directly caused the outcome (dementia).

There are lots of possible confounding factors. Although the researchers tried to account for some of them, there may be others that were missed.

But don't throw away your toothbrush. Good oral health is important for many reasons, and it may play a role in reducing the risk of dementia. What we don't know is how big a role, and whether it's a direct cause and effect.

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